With the prevalence of mobile devices, the existing 3rd generation (3G) and the 3.5 generation (3.5G) technologies no longer can support the continuous growth of wireless applications and services. Therefore, Long Term Evolution (LTE) was proposed by the 3rd Generation Partnership Project (3GPP) as a new network standard to solve this problem. After the LTE Release 10, LTE was further improved as Long Term Evolution-Advanced (LTE-A), which is regarded as the 4th generation (4G) standard. In LTE-A, new technologies including enhancements for diverse data applications (eDDA), multi-input multi-output (MIMO), carrier aggregation (CA), small cell and device-to-device (D2D) communication, are proposed to improve network capacity and efficiency.
Among these new technologies, D2D communication is considered as a key enabling technology to facilitate machine-to-machine (M2M) communication in LTE-A. In the future M2M communication, a sheer number of machines need to communicate with each other for diverse applications such as home or office automation, intelligent vehicles or transportation systems, or smart power monitoring. The resulting control and data traffic from these machines, if directly injected to the LTE network, will overwhelm the network and degrade the performance of existing human-to-human communications. With the help of LTE-A D2D communication, machines (i.e., user equipments (UEs) in LTE-A) in proximity can communicate directly and locally, and thus lessen their impact on the LTE infrastructure. In addition, machines themselves also benefit from D2D communication for shorter communication latency. Furthermore, higher data rates can be supported while less power is consumed due to better channel quality and shorter physical distance between machines in proximity.
D2D communication has been widely discussed in the 3GPP meetings. A study item “proximity-based services (ProSe)” is created on the 3GPP Technical Specification Group Service and System Aspects 1 (TSG-SA1) meeting #55, and several usage scenarios are identified. Although different scenarios have their own requirements, a common set of functionalities is always needed. For example, UEs in the proximity must be able to discover each other (i.e., peer discovery). In the existing LTE network, nearby eNB discovery is through synchronization signal (PSS/SSS), and UEs can only connect to them via the random access procedure on PRACH. Therefore, a new mechanism—possibly with the assistance from the eNB—is needed for peer discovery. Peer discovery is divided into two types depending on whether D2D UEs have an ongoing session or not. If D2D UEs do not have a session, UEs may need to broadcast signals to identify themselves, which can be regarded as beacons to let other UEs know their existence. Since UEs do discovery themselves, the influence on the core network is very little. This type of discoveries is more suitable for M2M. However, transmitting beacons is power consuming, which is a key concern for M2M, especially when UEs transmit beacons blindly.
FlashLinQ is a reservation-based peer discovery method which is not designed based on LTE-A. Devices using it need to be globally synchronized. The frequency bandwidth FlashLinQ uses is 5 MHz and one discovery repetition period is 8 seconds. Furthermore, resources are further divided into 3584 peer discovery resource IDs (PDRIDs) in one repetition. As a device enters the network, it senses the channel and chooses a PDRID with a low signal power for transmitting a beacon to avoid collisions. Then it listens for beacons in the rest of the repetition. The time that UEs transmit beacons in every repetition will shift differently based on different PDRIDs they choose. The purpose is to avoid the case that two half-duplex UEs always transmit at the same time and cannot discover each other.
Although FlashLinQ claims that a device can discover about one thousand devices in ten seconds, there are still some salient problems. First, since every device has to reserve a dedicated PDRID for itself, the free PDRIDs in different places are different. If two faraway devices choose the same PDRID, once they are close there is a collision. Although devices can detect collisions and reselect other PDRIDs, the probability they choose the same PDRID again is still high. Because they sense the same region, the optimal free PDRID they sensed is the same. Therefore, collisions in the case that devices have mobility will be too much for devices to deal with. The efficiency for discovery is thus low. Second, when a device enters the network, the waiting time for sensing the channel is long. This might cause collisions too. The probability that at least two nearby device is turned on for the discovery in one repetition is not low. After sensing, they may choose the same optimal free PDRID with very high probability and cause collisions.
A solution is sought on peer discovery for D2D communication in LTE-A networks.